1. Field of the Invention
This invention relates to a display device, specifically to a display device performing multiple gray level display corresponding to multi-bit digital video data.
2. Description of the Related Art
Organic EL device using an organic electroluminescence elements (hereafter referred to as organic EL element) have been receiving attention in recent years as a display device which would replace a CRT and an LCD. An active matrix type organic EL display device having a thin film transistor (hereafter referred to as a TFT) that serves as a driver transistor supplying a drive current to an organic EL element in each of pixels has been developed.
In a basic structure of the active matrix type organic EL display device, each pixel 52 includes an organic EL element 50 made of a part of an organic luminescent layer, a driver transistor TR2 that controls a current flow to the organic EL element 50, a write transistor TR1 that is turned on when a scanning voltage SCAN is applied to a scanning electrode and a capacitor C that stores electric charges when a data voltage DATA from a data electrode is applied. An output of the capacitor C is applied to a gate of the driver transistor TR2, as shown in FIG. 11.
First, the scanning voltage SCAN is successively applied to each of the scanning electrodes to turn on a plurality of the write transistors TR1 connected to a common scanning electrode. A data voltage (input signal) is applied to each of the data electrodes in synchronization with the scanning. Since the write transistor TR1 is turned on at that time, the data voltage DATA is stored in the capacitor C.
An amount of electric charges stored in the capacitor C by the data voltage determines a status of operation of the driver transistor TR2. When the driver transistor TR2 is put into an active status, for example, a current corresponding to the data voltage DATA flows to the organic EL element 50 through the driver transistor TR2. As a result, the organic EL element 50 emits light with brightness corresponding to the data voltage DATA. This light emitting status continues for a vertical scanning period.
A method to drive the organic EL element 50 to a brightness corresponding to the data voltage by proving the organic EL element 50 with the current corresponding to the data voltage as described above is called an analog drive method. On the other hand, an organic EL display device using a digital drive method with which multiple gray level display is implemented by providing the organic EL element 50 with a pulse current having a duty corresponding to the data voltage is proposed.
In the organic EL display device using the digital drive method, one field (or one frame) that is a period to display one screen of picture is divided into a plurality (N) of sub-fields (or sub-frames) SF, and each of the sub-field SF is composed of addressing periods (scanning periods) AP during which the data voltages are written into all of the pixels and light emitting periods LP during which the organic EL element 50 in each of the pixels emits light corresponding to the written data voltage, as shown in FIG. 13.
The light emitting periods LP included in a field vary in length to have lengths of 2n (n=0, 1, 2, - - -,N−1), while all the addressing periods AP in a field have the same length. In an example (N=8) shown in FIG. 13, eight sub-fields SF0-SF7 are included in one field. All of eight addressing periods AP0-AP7 have the same length. Each of eight light emitting periods LP0-LP7 is set to each of lengths 1, 2, 4, 8, 16, 32, 64 and 128, so that a display of 256 gray levels is available with turning on and off during each of the light emitting periods.
In a sub-field drive method described above, binary data of a sub-field SF is written into a capacitor C by applying a scanning voltage to a write transistor TR1 forming each of pixels 52 during a scanning period in each of the sub-fields SF, and a driver transistor TR2 provides an organic EL element 50 with a current corresponding to the binary data later in a light emitting period, as shown in FIG. 12. Starting time and ending time of light emitting period of each of the organic EL elements 50 in each of the sub-fields can be aligned by providing a current supply line to a driver transistor TR2 forming each of pixels 53 with a on/off switch SW in the sub-field drive method, as shown in FIG. 12. Related technical information is disclosed in Japanese Patent Publication Nos. 2003-241711, 2002-27847 and H10-312173.
When 2n levels of gray level display is performed with n bits of video data using the sub-field drive method described above, there arises a problem that fast addressing operation is required because the addressing period AP becomes shorter than (one field period/n) by tens of percents in order to secure a light emitting period LP. The light emitting period LP is not secured if the addressing period AP is set to be equal to (one field period/n). For example, assuming that a total addressing period to a total light emitting period in one field is 1:1 and that n is 8 and one field period is 16 msec, an addressing period in each of the sub-fields must be shorter than 16 msec/2/8=1 msec.